1. Field of the Invention
The present invention relates to a system for storing and dispensing liquid hydrogen without venting, and more particularly to such a storage and delivery system for use in an automobile, light or heavy duty trucks, boats, or transportation systems or vehicles in general that use hydrogen for fuel, such that a vehicle can store a large quantity of hydrogen fuel in the liquid state without requiring hydrogen venting due to boil-off.
2. Description of Related Art
Recently, and especially in view of the global oil climate, the automotive industry has reported serious development efforts to produce hydrogen powered vehicles. A key benefit to using hydrogen as opposed to conventional fossil fuels is that hydrogen burns cleanly, producing only water as a combustion product, it yields no carbon monoxide or carbon dioxide emissions, and it dramatically reduces NOx emissions. When used with fuel cells, hydrogen reacts with oxygen and produces only water. Furthermore, hydrogen fuel is abundantly available in limitless supply, whereas existing fossil fuel reserves are finite, and eventually will run out. For these reasons, hydrogen would seem to be an ideal fuel for modern automobiles. However, widespread use of hydrogen as automobile fuel has been prevented due to several major concerns.
In particular, storing and handling liquid hydrogen is difficult and can be dangerous. Hydrogen is highly combustible and is prone to explode when combined with atmospheric oxygen in the presence of an ignition source. In addition, hydrogen has a very high vapor pressure, boiling at about 20K under atmospheric pressure. Therefore, conventionally hydrogen is stored either in its gaseous state in high pressure tanks (about or even greater than 5000 psia gaseous hydrogen), or in low pressure liquid storage vessels which must be vented to relieve hydrogen overpressure as a result of vaporization due to ambient heat leak into the storage vessels (e.g. typical ambient temperature of about 298K or 25° C.).
Neither of these hydrogen storage mechanisms is suitable for use in privately owned and operated automobiles. First, at 5000 psia, conventional size high-pressure storage tanks can hold only a relatively small amount of hydrogen, i.e. about 2-4 kg. The size of these tanks is further limited due to space and weight constraints in an automobile. The limited storage volume for hydrogen fuel means limited automobile driving range between refuelings. Another perhaps more significant disadvantage is that storing a highly explosive fuel, such as gaseous hydrogen, at high pressure on the order of 5000 psia in an automobile presents a significant danger to both occupants and bystanders should the pressure tank or associated high pressure hardware fail.
Second, conventional low pressure liquid hydrogen storage vessels are unsuitable for storing hydrogen fuel in automobiles due to the requirement of venting vaporized hydrogen gas to the atmosphere. Hydrogen overpressure results from heat transfer from the environment into the storage vessel causing liquid hydrogen to boil and vaporize. The hydrogen vapor continues to expand as ambient heat energy is further absorbed, and the storage tanks must be vented of this hydrogen vapor to prevent explosion of the vessel due to hydrogen overpressure. Therefore, today's typical liquid hydrogen storage systems involve or require controlled venting of boil-off hydrogen gas whose release into the atmosphere must be directed and controlled to prevent concentrated pockets hydrogen gas in confined spaces (e.g. inside buildings) where it may explode or cause a fire. For these reasons, existing hydrogen vents typically are located in remote areas or at safe distances above the tallest structure in the vicinity of the vent. If the quantity of vented hydrogen is sufficiently large then burn-off stacks usually must be utilized.
It is easy to understand why it will be impractical for every parking space, garage, and driveway to be equipped with a hydrogen vent stack for venting hydrogen overpressure while a hydrogen-powered vehicle is parked. In fact, even if it were possible to provide such stationary hydrogen vent stacks, automobiles are by definition “mobile,” and it is totally impractical to limit people to stationing their cars only in locations where there is an available hydrogen vent stack. In addition, the near continuous venting of hydrogen gas is a significant waste of fuel that greatly diminishes the efficiency of a hydrogen-powered automobile.
Several automotive manufacturers have conducted experiments involving vehicles that operate using hydrogen in both liquid and gaseous forms, however no one has yet devised a mechanism for storing highly dense liquid hydrogen (compared to gaseous hydrogen) in a hydrogen-powered automobile which does not require venting to relieve hydrogen overpressure.
Another technology that has been proposed to store large quantities of hydrogen is surface-adsorption onto a metal-hydride matrix. It has been reported that hydrogen densities (mass hydrogen per storage volume) comparable to liquid hydrogen can be obtained via this technique of adsorbing onto the surface of metal hydride matrices, however the inventors have not been able to verify this. Regardless, there are still several disadvantages to the use of metal hydrides for storing gaseous hydrogen. First, storing hydrogen in this manner requires ultra high purity hydrogen gas; even very small amounts of common contaminants such as carbon monoxide would significantly decrease the metal hydride adsorptive storage capacity for hydrogen. Second, to date only very small metal hydride containers (e.g. for use in portable electronic devices) have been developed, and it is not clear that extrapolating this concept to the scale required to store a reasonable quantity of hydrogen fuel for an automobile would be straight-forward or even possible. Third, the exotic metals required to produce metal hydride matrices for hydrogen storage would be cost prohibitive on the scale required to provide adequate hydrogen storage capacity for an automobile. Fourth, metal hydride hydrogen adsorption systems are very heavy, and may contribute significantly to the weight of an automobile.
Accordingly, there is a need in the art for a system for storing hydrogen fuel in liquid form in a hydrogen-powered automobile or in an automobile equipped with a hydrogen powered fuel cell, that does not need to be vented to relieve hydrogen overpressure.